Aerosols



United tates atent 2,980,582 AEROSOLS John Lewis Keats, Wilmington, Del., assignor to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware No Drawing. Filed Apr. 7, 1945, Ser. No. 587,217 7 Claims. Cl. 167-39) This invention relates to improved methods for producing dispersions of finely divided solids in air. I 7 Solid aerosols, dispersions of finely divided solids in air, have previously been prepared by various methods. 7

terial and being under a pressure of at least 20 atmospheres in said closed receptacle. 7

One preferred embodiment of this invention'comprises bursting in the air a closed receptacle containing carbon dioxide under a pressure of at least 20 atmospheres and a protein, e.g., blood albumin, egg albumin, gliadin, zein, casein, or the like, having a particle size of less than 10 microns weight median diameter. 5

Another preferred embodiment of this invention comprises bursting in the air a closed receptacle containing carbon dioxide under a pressure of at least 20 atmospheres and an insecticide, 'e.g., pyrethrum, rotenone, ferric dimethyl dithiocarbamate, p,p'-.dichlorodiphenyltri chloroethane, dodecyl thiocyanate, lead arsenate',-calcium arsenate, sulfur, basic copper carbonate,'barium fluosilicate, calcium fluosilicate, or the like, having a particle size of less than 10 microns weight median diameter.

Various arrangements and selections of equipment for the operation of my process are. possible. .In one preferred arrangement the solid material to be dispersed having a particle size of less than 10 microns weight median diameter, said material being treated with about 1% 'of its weight of a surface-active agent which reduces cohesion between the particles, is placed within a container capable of being ruptured. A low-boiling liquid or gas which is a non-solvent for the finelydivided solid 7 material and which has a vapor. pressure of. at leastZO air. This method is satisfactory only for dispersing persed. A still further object is the provision of a process for generating solid aerosols in diflicultly accessible locations. Additional objects will become apparent atmospheres at 25C,, isthen placed under a pressure of at least 20 atmospheres in the closed container. The charged container after being placed in thelocality where the aerosol is to begenerated, is burst or ruptured-or 1 shattered by appropriate means whereby theffinelygdivided solid is dispersed into an aerosol bytlieresultant single, endothermic, disruptive burst of the compressed gas or low-boiling liquid. P

The following examples, in which proportions tram parts by weight unless otherwise specified, are given for illustrative purposes and are not intended to place any from an examination of the following description and claims.

These and other objects and advantages are accomplished according to the hereindescribed invention which broadly comprises the process for the preparation ofvsolid aerosols by subjecting a finely divided solid material to a single, explosive, disruptive burst in which no heat is generated. In a more restricted embodiment this invention comprises bursting in the air a recepacle containing a finely divided solid material and a fluid having a vapor pressure of at least three atmospheres at 2 5 C., said fluid having the property of expanding endothernially, being a non-solvent for said solid material and being under a pressure of at least three atmospheres in said receptacle.

In a still more restricted embodiment this invention comprises bursting in the air a receptacle containing a solid material having a particle size of less than 20 microns weight median diameter and a fluid having a vapor pressure of at least 10 atmospheres at 25 C., said fluid having the property of expanding endothermally, being a non-solvent for said solid material and being under a pressure of at least 10 atmospheres in said receptacle.

Ina still more restricted embodiment this invention comprises bursting in the air a closed receptaclecontaining a solid material having a particle size of less than 10 microns weight median diameter and a fluid having a vapor pressure of atleast 20 atmospheres at 25 C., said fluid having the property of expanding endothermally, being a non-solvent for said solid marestrictions on the hereindescribed invention.

A cylindrical steel vessel fitted with a capillary" inlet for gas and a shear'disc head and having a capacityof about 7.5 cc. is charged with 1.98. gramsof egg albumin which has been ground in a ball mill until the particles have a weight median diameter of 6.6 microns and 76% (by weight), have a diameter of less than'10 microns.

The opening in the head is closed by discs ofcellulose acetate of sufficient aggregate thickness to require a pressure of about 700 lbs./sq.'in. to rupture them. Car bon dioxide is admitted through the capillary inletuntil 7 the internal pressure is suflicient to rupture the closure.

The rupture of the closure is accompanied by an explosive eruption of the contents with the formation of an egg albumin aerosol without the generation of heat. De termination of vthe mass concentration of solid particles in the aerosol carried out on three concordant runs, by the method described below, indicates that on'an average 64%, 39% and 27% of the theoretical amount (the total weight of the solid originally used for the preparation of the aerosol) of egg albumin remains suspended in the air for periods of one to five, six to ten and eleven to fifteeii minutes, respectively, after therupture of the vesse The aerosol is generated in a closed chamber approXimately 10 ft. x 10 ft. x 10 ft. in dimensions at a point approximatelyZ feet below the geometric center of the chamber. A circulating fan placed directly below the with Pyrex glass wool carefully placed to eliminate channels without packing to the point of high resistance,

each of which has been dehumidified by drawing through i i it over an eight-minute period 1.4 cu. ft. of air which hasbeen dried by calcium chloride. These tubes are arranged inthreesets of three each on a verticalframe which stands two feet away from the center of the hour of the chamber. In each set of three, one tube is placed horizontally ateach of three levels g/ ft, 5 ft. and 8 ft.) from the floor of the chamber. Each set of three tubes (A, B and. C) is attached to a branching vacuum line of rubber tubing, the single end of which leads of a cubic 'foot upward. The vacuum source is an air pump with capacity of 3 cu ft. of air per minute. A differential manometer is attachedt'o the vacuum line side of the meter sothat the meter reading can be corrected to atmospheric pressure. Exactly one minute after the generation of the aerosol the circulating fan is stopped "and filtration of the aerosol through. Set A of the filt'e'r tubes is begun. Sampling through Set A is'termin'ated at 5 minutes and Set B and Set C are usedin the same way during the intervals six to ten. minutes, and eleven to fifteen'minutes, respectively; after. the. generation of the plodes violently producing a cloudof smoke composed aerosol. Gas meter readings are obtained before and after each filter period and manometer readings during each one. The filter tubes are again dehumidified before removal from the filter positions by drawing dry air through all nine tubes at the rate of 1.6 cu. ft. per

minute for eight minutes. The tubes arethen removed and stoppered, carefully wiped with a clean chamols,

and weighed. In calculating. the mass concentration of the aerosol from these measurements, the volume of air passed through the filter is first calculated by the formula: a

V0: zsavx are based is:

do C2) wherein C is the mass concentration, v is the velocity of sedimentation calculated from Stokes law, I is the time and h is the height of the chamber. By combining the integrated form of this equation with Stokes formula the following expression for the average diameter d is obtained:

[181 b In C C d= wherein 1; is the coefiicient of viscosity of the gaseous medium, h is the height of the chamber, C and C are the average mass concentrations of the solid in the gaseous medium a and b minutes, respectively, after the burst, g is the acceleration of the solid body due to gravitation, p is the density of the solid material and lis the time. Using the values 1 (viscosity of air)=1.86 10- poises, h=250 cm., g (acceleration)=981 cm. per sec. per sec.,

4 the follovw'ng expression for theaverage diameter D in microns is obtained:

where C is the average mas's concentration of solid in air during the period one to fiveminutesafter the burst,

C is the average mass concentration of solid in airfor 1 the period eleven to fifteen minutes after the burst; and p is the density of the solid. i

, Example II Aglass ampoule having a volume of about 50 cc. is

loaded with 22 grams of solid carbon dioxide and the tube evacuated and sealed. The cold ampoule is placed in an axial position in the center of a paper cylinder containing 22 grams of finelydivided lithopone, havingan average particle size of about 0.5 micron diameter and which has, been coated with 1% of its weight'of the cock:-

nut oil soap available commercially unde'rthe trade name Distolene. The lower end of the glass ampoulcprojects below the end of the outer paper cylinder and the upper portion of the ampoule projects above thepaper cylinder.

'Vanes' are attached to the upper end of the ampouleto control. the direction in which it falls in air. After allowingithe container to warm up to room temperature it isall'owed to drop onto a hard surface, whereupon it exof finely divided lithopone.

Example III One thousand grams of zinc oxidehaving an average particle sizeof about 0.2 micron diameter, which is available commercially as a pigment grade of zinc oxide, 5000 g. of water and 10 g. of diethylene glycol monolaurate arepassed through a colloid mill until a uniform slurry is. obtained, fifteen minutes beingrequired. Theresulting surface treated zinc oxide is filtered from the water, dried at 115 C. and passed through a Micronizer to disintegrate the filter cake into individual solid particles. The gas bombwith she'ar disc head of Example I is charged with 2.83 g. of the resultant zinc oxide having a particle size of about 0.2 micron diameter surface treated with 1% of its weight of diethylene glycol rnonolaurate. After closing the bomb with cellulose acetate discs, carbon: dioxide gas is introduced until the internal pressure is sulficient to rupture the cellulose acetate closure, this being about 700 lbs/sq. in. Mass concentration data obtained for the zinc oxide aerosol produced indicate that, on the average, 32%, 26% and 17% of the theoretical amount of solid remains airborne for periods of one to five, six to ten and. eleven to fifteen minutes, respectively, after the generation of the aerosol.

In addition to the specific methods of dispersing finely divided solidsdescribed in theexamples, other modifications of the general process of this invention. involving the use of a single, endothermic, disruptive burst may-be used. For example, the finely divided solid particles may be placed in a container made of glass or other frangible material together with solid carbon dioxide, the container closed and the contents allowed to come to room tem: peratu're with liquefaction ofthe carbon dioxide, and the i vessel thenrup'tur'ed by'external means.

have a particle size of less than 20 microns weight median diameter. Optimum results are obtained when said material has a particle size of less than 10 microns weight" median diameter. By weight median diameter, as employed herein and in the appended claims, it is meant that the amount by weight of the solid material particles having a diameter less than the specified diameter equals the amount by weight of said particles having a diameter greater than said specified diameter.

The dispersibility of the solid particles may be improved, if desired, by treating them with a material which reduces cohesion between them, e.g., by coating said particles with about 1% by weight of a surface active material. Examples of such surface-treating agents which are particularly etfective include diethylene glycol monolaurate, mannitan monolaurate and soya lecithin. The presence of such surface-treating agents is not, however, essential to the production of aerosols according to the method of this invention.

While this invention has been illustrated with particular reference to the use of carbon dioxide as dispersant, it is contemplated that any substance which is a non-solvent for the solid material being dispersed, which has a vapor pressure of at least three atmospheres at 25 C., and In other stances are carbon dioxide, carbon monoxide, ammonia,

sulfur dioxide, nitrogen, acetylene, oxygen, air, ethane, ethylene and methyl chloride, it being understood, of course, that in every instance the substance used to provide the disruptive force is one which is inert toward, and a non-solvent for, the finely divided solid being dispersed. As hereinbefore stated, the substance employed as a vehicle for the finely divided solid, i.e., the substance used to provide the endothermic disruptive burst, should have a vapor pressure of at least three atmospheres at 25 C. Markedly improved results are obtained when said substance has a vapor pressure of not less than ten atmospheres at 25 C., while optimum results are had when said substance has a vapor pressure of twenty atmospheres or more at 25 C.

The disruptive substance contained in the receptacle should be under a pressure of at least three atmospheres at the moment of bursting. Improved results are obtained when said substance is under a pressure of not less than 10 atmospheres, while maximum results are had when said substance is under a pressure of at least atmospheres at the moment of the bursting of said receptacle. The only known maximum pressure is that which the container is capable of withstanding. The optimum pressure employed in any given instance depends to a marked degree on the particular type of solid being dispersed, a light flufiy solid requiring less pressure than a more compact one.

Because of the endothermic nature of the disruptive force of this invention, it is particularly useful for the generation of aerosols of finely divided solids which are susceptible todecomposition by heat. It is particularly useful for the preparation of aerosols composed of proteins or insecticides.

As many apparently widely different embodiments of this invention may be made without departing from the I spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

Having described the present invention, the following is claimed as new and useful.

l. The process for obtaining an aerosol, which comprises bursting in the air a frangible receptacle containing a finely divided solid material and a fluid, said fluid in liquid phase having a vapor pressure of at least three atmospheres at 25 C., having the property of expanding endothermally, being inert toward, and a non-solvent for, said solid material and being under a pressure of at least three atmospheres in said receptacle.

2. The process for obtaining an aerosol, which comprises bursting in the air a frangible receptacle containing a solid material having a particle size of less than 20 microns weight median diameter and a fluid, said fluid in liquid phase having a vapor pressure of at least l0 atmospheres at 25 C., having the property of expanding endothermally, being inert toward, and a non-solvent for,

said solid material and being under a pressure of at least 10 atmospheres in said receptacle.

3. The process for obtaining an aerosol, which comprises bursting in the air a closed frangible receptacle containing a solid material having a particle size of less than 10 microns weight median diameter and a fluid, said fluid in liquid phase having a vapor pressure of at least 20 atmospheres at 25 C., having the property of expanding endothermally, being inert toward, and a non-solvent for, said solid material and being under a pressure of at least 20 atmospheres in said closed receptacle.

4. The process according to claim 3 wherein the solid material comprises an insecticide.

5. The process according to claim 3 wherein the solid material comprises a pigment.

6. The process according to claim 3 wherein the solid material comprises a protein.

7. The process for obtaining an aerosol, which comprises bursting in the air a closed frangible receptacle containing carbon dioxide under a pressure of at least 20 atmospheres and a solid material having a particle size of less than 10 microns weight median diameter, said solid material being inert toward and insoluble in carbon dioxide.

References Cited in the file of this patent OTHER REFERENCES Science News Letter, pp. 117-118, February 19, 1944.

Science News Letter, page 57, column 3, July 22, 1944.

Journal of Economic Entomology, volume 37, No. 1, page 136, column 2. 

1. THE PROCESS FOR OBTAINING AN AEROSOL, WHICH COMPRISES BURSTING IN THE AIR A FRANGIBLE RECEPTABLE CONTAINING A FINELY DIVIDED SOLID MATERIAL AND A FLUID, SAID FLUID IN LIQUID PHASE HAVING A VAPOR PRESSURE OF AT LEAST THREE ATMOSPHERES AT 25*C., HAVING THE PROPERTY OF EXPANDING ENDOTHERMALLY, BEING INERT TOWARD, AND A NON-SOLVENT FOR, SAID SOLID MATERIAL AND BEING UNDER A PRESSURE OF AT LEAST THREE ATMOSPHERES IN SAID RECEPTACLE. 